In such a circuit of a hydrostatic transmission with a closed circuit, for instance known from "Olhydraulik" , G. Bauer, 3rd edition 1982, B. G. Teubner Stuttgart, pressure control valves are provided which, when a certain pressure is exceeded, enable a bypass connection and thus a limitation of the torque of the hydraulic motor. The maximum limitation of the torque of a hydraulic motor is for instance necessary when a transmission is connected after the hydraulic motor. Said transmission can for instance be used for the drive of the wheels of a vehicle.
If the hydraulic motor is only used for driving a vehicle or another device with only one direction of rotation, the torque delivered by the engine in the accelerating phase and transmitted to the transmission can be limited in a simple manner by a corresponding design of the pressure control valve for the maximum permissible torque. Such a limitation is also then possible without any difficulties, when the hydraulic motor can be driven with both directions of rotation and can deliver power in both directions of rotation. In this case, the pressure line as well as the return line, which change their functions dependent on the direction of rotation, are to be secured by relief valves.
Hydrostatic hydraulic motors do not only operate as drive elements but very often they take over the function of engine brakes, when from the power take-off side a torque is transmitted via the transmission onto the engine, i.e. for instance when the amount of hydraulic oil supplied to the hydraulic motor corresponds to a torque of the engine which is less than the torque placed to the engine via the transmission, so that the hydraulic motor is operated as an engine brake in the pump mode. If, however, the hydraulic motor is operated as a pump in the braking mode, the return line becomes the pressure line in which a pressure can build up due to the securing by the pressure control valve, said pressure corresponding to the pressure in the supply line in the accelerating phase with maximum possible torque. Since, corresponding to its hydraulic efficiency, the hydraulic motor has losses during operation as motor as well as during operation as pump, the torque during pump mode at the engine shaft is much larger than during motor mode, when the pressure in the return line is of the same degree as during motor mode in the supply line. Thus, if the pressure in the return line during pump mode is limited by the relief valve to the same maximum pressure as during motor mode, due to the higher torque for instance the transmission will be overloaded. In order to render possible such an overload of the transmission without damaging it, said transmission is usually overdimensioned, in fact for instance by approximately 50 to 70 percent. Said overdimensioning of the drive parts does, however, lead to an increase in price of the vehicles or devices.
Therefore it is the object of the invention to develop a hydraulic circuit of the kind mentioned hereinbefore which limits the maximum torque at the shaft of the hydraulic motor to approximately identical values independent of the fact whether the hydraulic motor is operated in the motor mode or in the pump mode.